B-string half step sharp

I recently aquired a 5-string fretted neck-thru, with Fishman piezo bridge, and I restrung it with TI Powerbass strings.
I was able to intonate the first four strings nicely, the nut-to-twelfth-fret measurement (and consequently the twelfth-fret-to-saddle) approx 17.5 inches on each. Then I came to the low B string... I set the 12-to-saddle at 17.5 inches, and according to my Korg DT-3 it is almost a HALF STEP SHARP.
For the heck of it I tried to lengthen the string, I even removed the spring for some extra adjustment room, adjusted the saddle ALL THE WAY toward the bridge, and I was not able to get within a QUARTER STEP of low B.
I would appreciate ANY input that you might have, thanks!